Booroola BMPR1B mutation alters early follicular development and oocyte ultrastructure in sheep

Booroola ewes homozygous (BB) for a mutation in the bone morphogenetic protein receptor-1b (BMPR1B) gene exhibit higher ovulation rates, have larger diameter oocytes at earlier stages of follicular development (i.e. Type 3) and smaller diameter follicles at ovulation than wild-type (++) sheep. However, it is not known when BMPR1B is first expressed in the developing ovary or the cell types involved. In addition, the effects of the BMPR1B mutation on primordial (Type 1) follicles or during growth to the Type 3 stage are unknown. In the present study, BB and++fetal ovaries at Days 30-135 of gestation were screened by in situ hybridisation for BMPR1B mRNA. Ovaries from BB and++lambs were examined by microscopy to measure follicular and oocyte ultrastructural characteristics in Type 1-3 follicles. BMPR1B mRNA was observed in ovaries from Day 35 of gestation and was evident in oocytes of newly forming and fully formed Type 1 follicles. In BB animals, the Type 1 follicles had larger mean follicular and oocyte diameters, a greater volume of mitochondria, smooth endoplasmic reticulum and ribosomes and a greater surface area of junctions with the granulosa cells compared with++animals. It is concluded that the BMPR1B mutation alters follicular development from the onset of follicular formation.

The role of IGFs in the regulation of ovarian follicular growth in the brushtail possum (Trichosurus vulpecula)

IGFs are known to be key regulators of ovarian follicular growth in eutherian mammals, but little is known regarding their role in marsupials. To better understand the potential role of IGFs in the regulation of follicular growth in marsupials, expression of mRNAs encoding IGF1, IGF2, IGF1R, IGF-binding protein 2 (IGFBP2), IGFBP4 and IGFBP5 was localized by in situ hybridization in developing ovarian follicles of the brushtail possum. In addition, the effects of IGF1 and IGF2 on granulosa cell function were tested in vitro. Both granulosa and theca cells synthesize IGF mRNAs, with the theca expressing IGF1 mRNA and granulosa cell expressing IGF2 mRNA. Oocytes and granulosa cells express IGF1R. Granulosa and theca cells expressed IGFBP mRNAs, although the pattern of expression differed between the BPs. IGFBP5 mRNA was differentially expressed as the follicles developed with granulosa cells of antral follicles no longer expressing IGFBP5 mRNA, suggesting an increased IGF bioavailability in the antral follicle. The IGFBP protease, PAPPA mRNA, was also expressed in granulosa cells of growing follicles. Both IGF1 and IGF2 stimulated thymidine incorporation but had no effect on progesterone production. Thus, IGF may be an important regulator of ovarian follicular development in marsupials as has been shown in eutherian mammals.

Gene expression and secretion of LH and FSH in relation to gene expression of GnRH receptors in the brushtail possum (Trichosurus vulpecula) demonstrates highly conserved mechanisms

In eutherian mammals, the gonadotrophins (LH and FSH) are synthesized and stored in gonadotroph cells under the regulation of multiple mechanisms including GnRH. Very little is known about the regulation of gonadotrophin secretion and storage in pituitary glands of marsupials. This study revealed, using quantitative PCR and heterologous RIA techniques, thatLHBmRNA expression levels remained constant over the oestrous cycle, regardless of the presence of a preovulatory LH surge, which is characteristic of a hormone secreted under regulation. Our sampling regime was unable to detect pulses of LH during the follicular phase, althoughGNRHRmRNA levels had increased at this time. Pulses of LH were, however, detected in the luteal phase of cycling females, in anoestrus females and in males. There was a positive correlation between gene expression ofFSHBand plasma levels of FSH at different stages of the oestrous cycle and no pulses of FSH were detected at any time; all characteristics of a hormone secreted via the constitutive pathway. Usingin situhybridisation and immunohistochemistry methods, we determined that mRNA expression ofLHBandFSHB, and protein storage of gonadotrophins exhibited a similar pattern of localisation within the pituitary gland. Additionally, sexual dimorphism of gonadotroph populations was evident. In summary, these findings are similar to that reported in eutherians and considering that marsupial evolution diverged from eutherians over 100 million years ago suggests that the regulation of gonadotrophins is highly conserved indeed.

Expression of mRNAs encoding oestrogen receptor (ER) α and ERβ, androgen receptor and progesterone receptor during gonadal and follicular development in the marsupial brushtail possum (Trichosurus vulpecula)

The objective of the present study was to determine which ovarian cells express mRNAs for oestrogen (ERα and ERβ), androgen (AR) and progesterone (PR) receptors during ovarian and follicular development in the brushtail possum. Expression of ERα and/or ERβ mRNA was observed from birth, initially in cells of the blastema, then in the medullary cords from Day 20. ERα was expressed in the oocytes and granulosa cells of secondary and antral follicles. Preovulatory follicles did not express ERα mRNA, although their oocytes were not examined for any gene. ERβ mRNA was observed in oocytes at all follicular stages examined, but was not consistently observed in granulosa or theca cells. Expression of AR mRNA before Day 40 was very faint; thereafter, expression was observed in the medullary cords, peaking between Days 60 and 120. Oocytes, granulosa cells and theca of secondary and antral, but not preovulatory, follicles expressed AR mRNA. PR mRNA was expressed throughout the gonad by Day 20. Granulosa cells of some secondary and antral follicles and theca of antral follicles expressed PR mRNA. Thus, the expression of mRNAs encoding steroidogenic receptors in a time- and cell-specific manner supports a role for steroids in the process of ovarian follicular formation and growth.

The role of bone morphogenetic proteins 2, 4, 6 and 7 during ovarian follicular development in sheep: contrast to rat

The intraovarian roles of BMP family members such as BMP2, 4, 6 and 7 are not well understood, particularly in species with low ovulation rates such as sheep. Therefore, the objectives of these experiments were to determine the expression patterns of mRNAs encoding BMP2, 4, 6 and 7 during ovarian follicular development in sheep, and to determine the effects of these growth factors on ovine granulosa cell functions in vitro. For comparative purposes, the effects of these BMPs were also determined in rat granulosa cells since these factors have been most widely studied in this poly-ovulatory species. As assessed by in situ hybridization, non-atretic ovine follicles expressed mRNA for BMP6 but not 2, 4 or 7. Furthermore, expression of BMP6 was limited to the oocyte of primordial as well as primary, pre-antral and antral follicles. Reverse transcription-PCR of granulosa cell mRNA detected low levels of all the BMPs in some pools of cells. BMP2, 4, 6 and 7 each inhibited progesterone production from ovine granulosa cells without affecting cellular proliferation/survival. Similarly, these BMPs inhibited progesterone production from rat granulosa cells. However, they also stimulated cellular proliferation/survival of the rat granulosa cells highlighting a species-specific difference for these growth factors. In conclusion, in sheep, BMP2, 4, 6 and 7 inhibit granulosa cell differentiation without affecting proliferation. However, as BMP2, 4 and 7 were not detectable by in situ hybridization in any cells of non-atretic ovarian follicles, it seems unlikely that these proteins would have an important intra-ovarian role in regulating follicular development in sheep. In contrast, localization of BMP6 mRNA in the oocyte suggests that this BMP family member may have a paracrine and/or autocrine role in regulating follicular growth in sheep, as has been shown for two other oocyte derived from members of the transforming growth factor superfamily, BMP15 and growth differentiation factor 9.

The role of transforming growth factor-beta (TGF-beta) during ovarian follicular development in sheep

BACKGROUND

Recently, several members of the transforming growth factor-beta (TGF-beta) superfamily have been shown to be essential for regulating the growth and differentiation of ovarian follicles and thus fertility.

METHODS

Ovaries of neonatal and adult sheep were examined for expression of the TGF-betas 1-3 and their receptors (RI and RII) by in situ hybridization using ovine cDNAs. The effects of TGF-beta 1 and 2 on proliferation and differentiation of ovine granulosa cells in vitro were also studied.

RESULTS

The expression patterns of TGF-beta 1 and 2 were similar in that both mRNAs were first observed in thecal cells of type 3 (small pre-antral) follicles. Expression of both mRNAs continued to be observed in the theca of larger follicles and was also present in cells within the stroma and associated with the vascular system of the ovary. There was no evidence for expression in granulosa cells or oocytes. Expression of TGF-beta 3 mRNA was limited to cells associated with the vascular system within the ovary. TGFbetaRI mRNA was observed in oocytes from the type 1 (primordial) to type 5 (antral) stages of follicular growth and granulosa and thecal cells expressed this mRNA at the type 3 (small pre-antral) and subsequent stages of development. The TGFbetaRI signal was also observed in the ovarian stroma and vascular cells. In ovarian follicles, mRNA encoding TGFbetaRII was restricted to thecal cells of type 3 (small pre-antral) and larger follicles. In addition, expression was also observed in some cells of the surface epithelium and in some stromal cells. In granulosa cells cultured for 6 days, both TGF-beta 1 and 2 decreased, in a dose dependent manner, both the amount of DNA and concentration of progesterone.

CONCLUSION

In summary, mRNA encoding both TGF-beta 1 and 2 were synthesized by ovarian theca, stroma and cells of the vascular system whereas TGF-beta 3 mRNA was synthesized by vascular cells. Luteinizing granulosa cells also responded to both TGF-beta 1 and beta 2 in vitro. These findings in sheep are consistent with TGF-beta potentially being an important autocrine regulator of thecal cell function and possibly a paracrine regulator of ovarian cell function at various development stages.